Method for operating an internal combustion engine

ABSTRACT

In a method for operating an internal combustion engine, the engine includes a plurality of cylinders each of which having at least one valve of a first type, an inlet valve, and at least one valve of a second type, an exhaust valve, arranged in an end part of the cylinder, each of the cylinders further having a piston movably arranged therein, the piston being adapted to reciprocate between a first end position and a second end position, wherein the first end position is located closer to the valves than the lower end position, the engine being adapted to allow deactivation of at least one cylinder by reducing its supply of fuel, the engine further being adapted to allow varying opening times of the inlet valves and/or the exhaust valves. The method includes extending the total opening time of at least one valve type of a deactivated cylinder compared to the opening time of the same valve type when the cylinder is active, the extension of the total opening time being performed such that at least one valve of the type is open at least partly both during a first engine stroke, wherein the piston moves in a direction towards its second end position, as well as during a second consecutive engine stroke, wherein the piston moves in a direction towards its first end position.

The invention relates to a method for operating an internal combustionengine that runs with one or several cylinders deactivated. Inparticular, the invention relates to a method for operating a dieselengine running with both active and deactivated cylinders.

Internal combustion engine exhaust gas after-treatment systems normallyneed to be operated within a certain temperature interval. Occasionally,the engine operating conditions are such that the exhaust gastemperature becomes too low for the exhaust gas after-treatment systemto work properly. Typically, this is a problem that can arise during lowload operation conditions of a diesel engine.

One way of increasing the exhaust gas temperature is to use a gasrestrictor positioned either on the exhaust side, in order to increasesthe engine load, or on the inlet side, in order to reduce the amount ofair entering the cylinders. This approach has, however, somedisadvantages: in the former case it leads to increased fuel consumptionand in the latter case it may, at least for a diesel engine, lead tomisfire and normally a new component will be required.

As described in e.g. U.S. Pat. No. 6,668,546 another way of increasingthe exhaust gas temperature during cold start and low load operatingconditions is to deactivate one or several cylinders by interrupting thefuel injection to these cylinders and increase the amount of fuelinjected in the cylinders that remain active. This way the exhaust gastemperature of the active cylinders will increase. When a certaincylinder is deactivated cool air will flow through it from the inletside to the exhaust side if the valves are operated in the same way asthe valves of an active cylinder. Such a cool gas flow will decrease theexhaust gas temperature. If the concept of cylinder deactivation is tobe used it is thus important to prevent or at least decrease a flow ofcold gas (air) via the deactivated cylinder(s).

A conventional solution to this problem for diesel engines is todeactivate the inlet valves of deactivated cylinders so that they aremaintained closed. This effect can be achieved with an arrangementcommonly referred to as variable valve movements of the “lost motion”type. The result is that the cool air is prevented from flowing throughthe cylinder as it is not allowed to even enter the cylinder. An effectof this solution is that the pressure in the deactivated cylinderbecomes very low during periods of the engine cycles, in particular atthe end of the intake stroke/beginning of the compression stroke, and atthe end of the expansion stroke (before the exhaust valve opens). Thiseffect has the disadvantage that, at least when the piston is relativelyclose to its bottom dead center (BDC) position, the cylinder pressurebecomes lower than the crank case gas pressure which pose a risk thatcrank case gas will be transferred from the crank case to the cylinder.Crank case gas contains oil that, if transferred to the cylinder,subsequently will be blown out to the exhaust gas after-treatment systemand give rise to increased emission levels and degradation of thecatalytic converters. In addition, this leads to an increasedconsumption of engine oil. The risk of transferring oil to the cylinderis in particular a problem for diesel engines that normally are notequipped with piston rings adapted to prevent transfer of crank gas.

U.S. Pat. No. 6,161,521 discloses another variant of avoidingthrough-flow of air for Otto-engines where camshaft timing is used tocontrol the valves, and thus the airflow, during so-called decelerationfuel shut off (DFSO) where all cylinders are deactivated at the sametime. This variant is, however, not easy to apply to a diesel enginesince the camshaft(s) normally can not be phased to the same extent asit involves having the valves open fully, or at least to a large extent,when the piston is at the top dead center (TDC) position which wouldlead to interference between the valves and the piston. In a dieselengine, working with a much higher compression ratio than an Ottoengine, the distance above the piston at TDC is much shorter. Moreover,it is not clear from U.S. Pat. No. 6,161,521 how the principles shouldbe applied in a case where some cylinders are supposed to be active.

From the above it follows that there is a need for improvements inpreventing airflow through deactivated cylinders of an internalcombustion engine, in particular with regard to diesel engines.

It is desirable to provide an improved method for operating an internalcombustion engine that runs with one or several cylinders deactivated.

An aspect of the invention concerns a method for operating an internalcombustion engine, said engine comprising a plurality of cylinders eachof which having at least one valve of a first type, an inlet valve, andat least one valve of a second type, an exhaust valve, arranged in anend part of the cylinder, each of said cylinders further having a pistonmovably arranged therein, said piston being adapted to reciprocatebetween an first end position and a second end position, wherein thefirst end position is located closer to the valves than the lower endposition, said engine being adapted to allow deactivation of at leastone cylinder by reducing its supply of fuel, said engine further beingadapted to allow varying opening times of the inlet valves and/or theexhaust valves. An aspect of the invention is characterized in that themethod comprises the step of extending the total opening time of atleast one valve type of a deactivated cylinder compared to the openingtime of the same valve type when the cylinder is active, said extensionof the total opening time being performed such that at least one valveof said type is open at least partly both during a first engine stroke,wherein the piston moves in a direction towards its second end position,as well as during a second consecutive engine stroke, wherein the pistonmoves in a direction towards its first end position.

Thus, the same valve, or at least the same type of valve, is open duringtwo consecutive engine strokes resulting in that gas or air that hasentered the cylinder during the first stroke will leave the cylinderduring the second stroke the same way it entered. Thereby no cool airwill pass through the deactivated cylinder. Further, the first enginestroke corresponds to either an intake stroke or an expansion stroke.Conventionally, the valves of a deactivated cylinder are closed duringthis first engine stroke since the inlet valve is maintained closed inthe conventional solution of the “lost motion” type, and since theexhaust valve normally is closed during this stroke. In contrast,according to the inventive method at least one valve is open during thisfirst engine stroke. An advantage according to an aspect of theinventive method is thus that no work is required for overcoming the lowpressure created in a cylinder with the valves closed during the firstengine stroke. This makes an engine operated according to the inventionmore efficient. A further advantage is that one avoids the situation ofhaving a very low cylinder pressure during the end of the first enginestroke which eliminates the risk of transferring oil from the crank caseto the cylinder.

In a first advantageous embodiment of an aspect of the invention themethod comprises the step of extending the total opening time of the atleast one inlet valve of a deactivated cylinder, wherein the firstengine stroke corresponds to an intake stroke and the second enginestroke corresponds to a compression stroke. Preferably, this is achievedby delaying the time of closure of the inlet valve of a deactivatedcylinder compared to the time of closure of the inlet valve when thecylinder is active, or by opening at least one inlet valve during thesecond engine stroke.

BRIEF DESCRIPTION OF DRAWINGS

In the description of the invention given below reference is made to thefollowing figure(s), in which:

FIG. 1 shows, in a schematic view, a representation of an internalcombustion engine to which the invention can be applied,

FIG. 2 shows, in a schematic view, a detail of the internal combustionengine according to FIG. 1, and

FIG. 3 shows, in a schematic view, movements of an inlet valve and anexhaust valve during conventional operation of an engine, as well asduring operation of an engine according to a first and a secondembodiment of the present invention.

DETAILED DESCRIPTION

FIG. 1 gives a schematic representation of an internal combustion engine10 in the form of a diesel engine. The engine 10 comprises an engineblock 11 having six piston cylinders 12 together with inlet manifold 13and exhaust manifold 14. Exhaust gases from the engine are fed via anexhaust line 15 to a turbine rotor 17 of a turbocharger unit 16. Aturbine shaft 18 drives a compressor wheel 19 of the turbocharger unit16, which by way of an intake line 20 compresses incoming air anddelivers it to the inlet manifold 13 via an air intercooler 21. Fuel isfed to each cylinder 12 via fuel injectors 9 (see FIG. 2). Exhaust gasesthat have passed through the turbocharger unit 16 are led onwards by wayof an exhaust line 22 to an exhaust gas after-treatment device 23.Further, the engine 10 has a system for returning exhaust gases to theintake side of the engine 10 as so-called EGR gas, via a pipeline 31.This line comprises an EGR valve 32 and an EGR cooler 33. An enginecontrol unit (not shown) containing e.g. control program is connected tovarious engine devices, such as the fuel injection system and the EGRvalve 32, as well as to a number of sensors (not shown) for determininge.g. engine speed and position of accelerator pedal, for controlling theengine with reference to input data. Although the figure illustrates asix-cylinder engine, the invention can also be used in conjunction withother cylinder configurations.

FIG. 2 shows, in a schematic view, one of the cylinders 12 of the engine10 according to FIG. 1. A piston 2 is reciprocably mounted in a cylinder12 wherein a connecting rod 6 connects the piston 2 to a crank shaft 8.The piston 2 is provided with piston rings 3. The cylinder 12 isprovided with an inlet valve 5 for allowing intake air to enter thecylinder 12 and an exhaust valve 7 for allowing combustion products toleave the cylinder 12. A fuel injector 9 is positioned on top of thecylinder 12.

The operation of the valves 5, 7 during normal, active operation of thecylinder could be controlled in a conventional mechanical way by themovement of a cam shaft (not shown). Alternatively, the valves may beelectronically controlled. To achieve a variable valve actuation asfurther described below one may e.g. use a more sophisticated mechanicalsystem, a conventional mechanical system in combination with a hydraulicarrangement, or electronically controlled actuators. An example of auseful system is disclosed in WO2004/005677.

A conventional operation of a four-stroke internal combustion engine,such as the one showed in FIGS. 1 and 2, can be described in thefollowing principal way:

I. Intake stroke (crank shaft angle 0-180°): The piston 2 moves from afirst, upper end position (top dead center, TDC) downwards, towards thecrank shaft 8, to a second, lower end position (bottom dead center,BDC). During this stroke the inlet valve 5 is open and the exhaust valve7 closed so that air flows into the cylinder 12.

II. Compression stroke (crank shaft angle 180-360°): The piston 2 movesfrom BDC to TDC with both valves 5, 7 closed as to compress the air.

III. Expansion stroke (crank shaft angle 360-540°): The valves 5, 7 arekept closed. Fuel is injected when the piston 2 is close to TDC. Thecombustion reaction between the fuel and the compressed air forces thepiston 2 towards BDC.

IV. Exhaust stroke (crank shaft angle 540-720°): The piston 2 moves fromBDC to TDC with the exhaust valve 7 open so that the exhaust gas ispushed out from the cylinder 12.

The terms intake, compression, expansion and exhaust stroke may not befully adequate when referring to a deactivated cylinder. Nevertheless,these terms are convenient also for describing the operation of adeactivated cylinder and should be interpreted as strokes correspondingto those of an active cylinder.

FIG. 3 shows, in a schematic view, movements of the inlet valve 5 andthe exhaust valve 7 during a conventional operation of an engine asdescribed above, as well as examples of movements of the inlet valve 5and the exhaust valve 7 during operation of an engine according to theinvention. The x-axis gives the crank shaft angle in degrees, whereasthe y-axis shows a valve lift in mm; the upper part of the y-axis refersto the inlet valve 5 and the lower part of the y-axis refers to theexhaust valve 7. It may be noted that the valve lift is a representationof how much the valve is open, e.g. if the valve lift is zero the valveis closed. The roman numerals I-IV refer to the four engine strokesdescribed above. The solid lines 51 and 71 correspond to the movementsof the inlet valve 5 and the exhaust valve 7, respectively, duringnormal operation of an internal combustion engine. These solid lines 51,71 thus represent both the normal valve movements of the cylinder 12when it is activated, as well as any active cylinder in an internalcombustion engine regardless of whether the engine is operating with anydeactivated cylinder(s) or not.

Referring now to the upper part of FIG. 3 the clashed line 52 correspondto the movements of the inlet valve 5 of a deactivated cylinder in aninternal combustion engine operating according to a first embodiment ofthe invention. The dashed line 52 is only shown where the valvemovements differ from the normal situation. As seen from the upper partof FIG. 3, the inlet valve 5 is opened according to normal operationduring the intake stroke but it is then maintained open, with a valvelift of around 4 mm, during most of the compression stroke. The inletvalve 5 is not closed until the piston 2 approaches TDC at the end ofthe compression stroke. The closure of the inlet valve 5 is thus delayedcompared to an active cylinder. As seen from the upper part of FIG. 3,the inlet valve 5 is open during a time period corresponding to a crankshaft angle interval of around 360°, wherein said time period includes apoint of time where the crank shaft angle is 180° at which the piston 2is in its lower end position (BDC). Thus, the inlet valve 5 is open bothduring a first engine stroke, the intake stroke I, wherein the piston 2moves towards the crank shaft 8, as well as during a second consecutiveengine stroke, the compression stroke II, wherein the piston 2 movesaway from the crank shaft 8.

Again referring to the upper part of FIG. 3 the dashed line 52′correspond to the movements of the inlet valve 5 of a deactivatedcylinder in an internal combustion engine operating according to avariant of the first embodiment of the invention. The dashed line 52′ isonly shown where the valve movements differ from the normal situation.As seen from the upper part of FIG. 3, the inlet valve 5 is opened andclosed according to normal operation during the intake stroke but it isthen opened again, up to a valve lift of around 4 mm, during thecompression stroke. At the valve lift of around 4 mm the two dashedlines 52 and 52′ coincide. Thus, again the inlet valve 5 is not closeduntil the piston 2 approaches TDC at the end of the compression stroke.Also in this case the inlet valve 5 is open both during a first enginestroke, the intake stroke I, wherein the piston 2 moves towards thecrank shaft 8, as well as during a second consecutive engine stroke, thecompression stroke II, wherein the piston 2 moves away from the crankshaft 8.

A result of this delayed closing, or additional post-opening, of theinlet valve 5 is that the air that has entered the deactivated cylinder12 during the intake stroke is free to leave, and will leave, thecylinder 12 via the open inlet valve 5 during the following compressionstroke. Since the inlet valve 5 is closed during the next two strokes,i.e. the expansion and exhaust strokes, no cool air will pass thecylinder and mix with, and thereby cool, the exhaust gas downstream thecylinder. An advantage of letting the inlet valve 5 be open during boththe intake and the compression strokes, compared to the conventionalsolution of the “lost motion” type where the inlet valve 5 is maintainedclosed, is that no work is required for overcoming the low pressurecreated in the cylinder 12 with the inlet valve 5 closed during theintake stroke. This makes an engine operated according to the inventionmore efficient. A further advantage is that one avoids the situation ofhaving a very low cylinder pressure during the end of the intake strokeand the beginning of the compression stroke which eliminates the risk oftransferring oil from the crank case to the cylinder during thesestrokes.

Referring now to the lower part of FIG. 3 the dashed line 72 correspondto the movements of the exhaust valve 7 of a deactivated cylinder in aninternal combustion engine operating according to a second embodiment ofthe invention. As above, the dashed line 72 is only shown where thevalve movements differ from the normal situation. As seen from the lowerpart of FIG. 3, the exhaust valve 7 is opened already at the beginningof the expansion stroke. After that it is maintained open during theexpansion stroke with a valve lift of around 4 mm and then,conventionally, held open during the exhaust stroke. The exhaust valve 7is thus opened shortly after the piston 2 leaves TDC at 360° between thecompression stroke and the expansion stroke. The opening of the exhaustvalve 7 is thus advanced compared to an active cylinder. As seen fromthe lower part of FIG. 3, the exhaust valve 7 is open during a timeperiod corresponding to a crankshaft angle interval of almost 360°,wherein said time period includes a point of time where the crankshaftangle is 540° at which the piston is in its lower end position (BDC).Thus, the exhaust valve 7 is open both during a first engine stroke, theexpansion stroke III, wherein the piston 2 moves towards the crank shaft8, as well as during a second consecutive engine stroke, the exhauststroke IV, wherein the piston 2 moves away from the crank shaft 8.

Again referring to the lower part of FIG. 3 the dashed line 72′correspond to the movements of the exhaust valve 7 of a deactivatedcylinder in an internal combustion engine operating according to avariant of the second embodiment of the invention. As above, the dashedline 72 is only shown where the valve movements differ from the normalsituation. As seen from the lower part of FIG. 3, the left parts of thetwo dashed lines 72 and 72′ coincide which means that also in this casethe exhaust valve 7 is opened already at the beginning of the expansionstroke, i.e. about at the point of time when the piston 2 leaves TDC at360° between the compression stroke and the expansion stroke. However,after a valve lift of around 4 mm the exhaust valve 7 is in this variantclosed again during a later stage of the expansion stroke whereupon theexhaust valve 7 is conventionally operated, i.e. open, during theexhaust stroke. Also in this case the exhaust valve 7 is open bothduring a first engine stroke, the expansion stroke III, wherein thepiston 2 moves towards the crank shaft 8, as well as during a secondconsecutive engine stroke, the exhaust stroke IV, wherein the piston 2moves away from the crank shaft 8.

A result of this advanced opening, or additional pre-opening, of theexhaust valve 7 is that a portion of exhaust gas is allowed to enter thecylinder 12 via the open exhaust valve 7 during the expansion stroke. Inthe next stroke, the exhaust stroke, this portion of gas will leave thecylinder 12 the same way it entered. An advantage of keeping the exhaustvalve 7 open during both the expansion and the exhaust strokes, comparedto the normal case where the exhaust valve 7 is open only during theexpansion stroke, is that no work is required for overcoming the lowpressure created in the cylinder with the exhaust valve 7 closed duringthe expansion stroke. In similarity to what is described above regardingthe inlet valve 5, this makes an engine operated according to theinvention more efficient. A further advantage is that one avoids thesituation of having a very low cylinder pressure during the end of theexpansion stroke which eliminates the risk of transferring oil from thecrank case to the cylinder at this stage.

Common to the embodiments and variants of the invention described aboveis that the valve, i.e. either the inlet valve 5 or the exhaust valve 7,is open both during a first engine stroke wherein the piston 2 moves ina direction towards the crank shaft 8, i.e. away from the valves 5, 7,as well as during a second consecutive engine stroke, i.e. an enginestroke following directly after the first engine stroke, wherein thepiston 2 moves in a direction away from the crank shaft 8, i.e. towardsthe valves 5, 7. A further common feature is that the total opening timeof the valve, i.e. either the inlet valve 5 or the exhaust valve 7, isextended compared to the opening time of the valve when the cylinder 12is active, i.e. during normal operation of the cylinder 12.

The inventive way of varying the movements of the inlet valve 5 asdescribed above may in principle be applied regardless of the movementsof the exhaust valve 7. However, to ensure that the amounts of cool airflowing through the cylinder 12 stays at a minimum the exhaust valve 7is preferably maintained closed at least during the compression stroke,which also is the case during normal operation of the exhaust valve 7.Conversely, the inventive way of varying the movements of the exhaustvalve 7 as described above may in principle be applied regardless of themovements of the inlet valve 5. If the movements of the inlet valve 5are carried out according to a normal operation of an active cylinder12, i.e. according to the solid line 51 in FIG. 2, there is no point inopening the exhaust valve 7 already in the expansion stroke with respectto air-flow through the cylinder 12. However, in such a case thetemperature of the air leaving the cylinder 12 via the exhaust valve 7has been increased due to compression in the former engine stroke. Thisheat of compression may be used to increase the temperature of theexhaust gas. If, on the other hand, the inlet valve 5 is maintainedclosed the above-mentioned advantages of the inventive way of varyingthe movements of the exhaust valve 7 will be achieved.

The most advantageous effect is, however, achieved by combining theinventive way of varying the movements of both the inlet valve 5 and theexhaust valve 7. This way all the abovementioned advantages regardingair-flow, efficiency and pressure difference will be carried intoeffect. As seen from FIG. 3 the inlet valve 5 and the exhaust valve 7are in principle not open at the same time which reduces the risk oftransferring cold air to the exhaust side.

The invention may be carried out with an internal combustion engine thatis adapted to allow deactivation of at least one cylinder by reducingits supply of fuel and that is adapted to allow varying opening times ofthe inlet valves and/or the exhaust valves. Valve movements or variablevalve actuation can for instance be controlled using electronicactuators, mechanical systems or a combined system involving a hydraulicarrangement. An example of an arrangement that may be used forcontrolling at least the inlet valves according to the invention isdisclosed in WO2004/005677. The arrangement disclosed comprises ahydraulic circuit that is used to vary the inlet valve closure duringthe intake stroke in order to switch between different operating modes.This arrangement could be modified as to delay the closure of an inletvalve of a deactivated cylinder until some point of time during thecompression stroke, thereby extending the time period during which thevalve is open.

In FIG. 3 the inventive method is exemplified with a valve lift of 4 mm.Further, the invention is exemplified with an extended valve openingtime of around 180°, as seen from the dashed lines 52 and 72, leading toa total opening time of around 360°, i.e. around two engine strokes, foreach type of valve. Still further, the invention is exemplified withpost- and pre-openings of around 90° as seen from the dashed lines 52′and 72′. These figures are only given as suitable figures for a givenengine and should not be interpreted as any type of limitation of theinvention. The advantageous effects of the invention are significant,and in many cases also sufficient, also with lower valve lifts andshorter opening times than shown in FIG. 3. Given the information inthis text it would be possible for a man skilled in the art to test outsuitable valve lifts and opening times for a particular engine.Regarding the first parameter, the valve lift, there is typically athreshold value that should be exceeded to get a sufficient flow of gasinto or out from the cylinder. This threshold value may be around 1 mm.Regarding the second parameter, the valve opening time, the advantageouseffects will, in principal, increase with the duration of the opening aslong as the valves are open during the adequate engine strokes asdescribed above.

The invention is not limited to the embodiments described above but canbe modified in various ways within the scope of the claims. Forinstance, the cylinder 12 may comprise more than one inlet valve 5and/or more than one exhaust valve 7. To achieve the advantages of theinvention it is sufficient that one inlet valve 5 and/or exhaust valve 7is controlled according to the invention. It is also possible to let aset of valves of similar type co-operate such that the combination of aparticular set of valves is operated according to the inventive methoddescribed above. For instance, one inlet valve 5 may be open during theintake stroke whereas another inlet valve 5 may be post-opened duringthe compression stroke.

The invention is focused on operation of deactivated cylinders in adiesel engine in a low-load situation with the main purpose ofincreasing the exhaust gas temperature, or at least decreasing thecooling of the exhaust gas, wherein one or several cylinders aredeactivated while one or several cylinders operate with an increasedfuel supply in a normal active mode. In such a case the inventive methodpreferably comprises one or several of the following steps: determininga representation of the engine load, determining a representation of theexhaust gas temperature, reducing the fuel supply to the cylinders to bedeactivated, and increasing the fuel supply to the active cylinders.However, the invention may be applied also in other situations such asduring DFSO of Otto engines.

Further, the invention is not limited to a situation where a certainnumber, or fraction, of cylinders is deactivated. For other reasons,such as to avoid engine vibrations, it may, however, be advantageous todeactivate a certain number of cylinders in a particular engine. Forinstance, for an engine with six cylinders it is normally an advantageto deactivate three cylinders instead of one. If such an engine isdivided into two cylinder banks one may use one of these banks fordeactivation and thus adapt only this bank to the inventive method.

The fundamental idea underlying the invention is that a valve of adeactivated cylinder is open at least partly during each of twoconsecutive engine strokes as to avoid a very low cylinder pressure, orat least decrease the number of low-pressure occasions, while at thesame time avoiding that cool gas (air) is transported via the cylinderto the exhaust gas after-treatment system.

In another embodiment of the invention the order of the valve opening ofthe deactivated cylinder 12 is reversed such that the exhaust valve 7 isopen during the intake stroke and/or the compression stroke and theintake valve 5 is open during the expansion stroke and/or the exhauststrokes.

1. A method for operating an internal combustion engine for increasingexhaust gas temperature, the engine comprising a plurality of cylinders,each of the cylinders having at least one valve of a first type, aninlet valve, and at least one valve of a second type, an exhaust valve,arranged in an end part of each of the cylinders, each of the cylindersfurther having a piston movably arranged therein, the piston beingadapted to reciprocate between a first end position and a second endposition, wherein the first end position is located closer to the valvesthan the second end position, the engine being adapted to allowdeactivation of at least one cylinder by reducing its supply of fuel,the engine further being adapted to allow varying opening times of atleast one of the exhaust valves, the method comprising: moving the firstand second valve types of a deactivated cylinder between open and closedpositions; increasing a fuel supply to at least one active cylinder, andextending a total opening time of at least one exhaust valve of adeactivated cylinder compared to an opening time of the same valve whenthe cylinder is active, the extension of the total opening time beingperformed such that the at least one exhaust valve is open at leastpartly both during an expansion stroke, wherein the piston moves in adirection towards its second end position, as well as during aconsecutive exhaust stroke, wherein the piston moves in a directiontowards its first end position.
 2. The method according to claim 1,comprising extending the total opening time of the at least one inletvalve of a deactivated cylinder compared to an opening time of the samevalve when the cylinder is active, the extension of the total openingtime being performed such that at least one inlet valve is open at leastpartly during both an intake stroke, wherein the piston moves in thedirection towards its second end position, as well as during aconsecutive compression stroke, wherein the piston moves in thedirection towards its first end position.
 3. The method according toclaim 1, wherein the engine is a diesel engine.
 4. The method accordingto claim 1, comprising advancing the opening time of the exhaust valveof a deactivated cylinder compared to the opening time of the exhaustvalve when the cylinder is active.
 5. The method according to claim 4,comprising extending the total opening time of the at least one inletvalve of a deactivated cylinder compared to an opening time of the samevalve when the cylinder is active, the extension of the total openingtime being performed such that at least one inlet valve is open at leastpartly during both an intake stroke, wherein the piston moves in thedirection towards its second end position, as well as during aconsecutive compression stroke, wherein the piston moves in thedirection towards its first end position.
 6. The method according toclaim 4, comprising opening at least one exhaust valve during theexpansion stroke.
 7. The method according to claim 6, comprisingextending the total opening time of the at least one inlet valve of adeactivated cylinder compared to an opening time of the same valve whenthe cylinder is active, the extension of the total opening time beingperformed such that at least one inlet valve is open at least partlyduring both an intake stroke, wherein the piston moves in the directiontowards its second end position, as well as during a consecutivecompression stroke, wherein the piston moves in the direction towardsits first end position.